1. Field of the Invention
The invention relates to a redox flow battery, in particular a vanadium redox flow battery, comprising at least two functional units, for example, at least two stages with at least one battery string, or at least two battery strings, wherein at least one unit is provided with a device for electrical decoupling, and a method to operate such a redox flow battery, whereby the not-needed functional units of which are deactivated.
2. The Prior Art
For redox flow batteries, the self-discharge in a stack is relatively high because of transport phenomena and shunt currents. To reduce self-discharge and hence to increase efficiency, multi-stage systems were implemented. Here, stacks which are not needed are “shut-down”, i.e., there is no electrolyte flow, and they are electrically decoupled. After stopping the flow, the charge remaining in the stack is transformed into heat due to self-discharge. Here, not only the energy loss is a problem, but also the temperature increase of the electrolyte.
JP 2003086228 A1 describes that also in case of stacks arranged above the tanks, and despite the stop of pumping the electrolyte, the whole amount of electrolyte does not flow out of the stacks. The part of electrolyte remaining in the stacks, whether in a charged or partially charged state, would self-discharge while generating heat. To solve this problem it is proposed to discharge the stacks with rest electrolyte into an external circuit by means of an inverter; however, for this a substantially constant load is required. Then, the tanks could also be arranged below the electrolyte in the stacks so that the pumping power can be reduced. During charging of the stacks, the circulation of the electrolyte does not start before the charge level of the electrolyte in the stack has reached the one of the electrolyte in the tank. However, a sub-stack remains always actively supplied with electrolyte, even when the flow in other stacks is stopped, to be able to cover an immediate electricity demand. Since it takes several minutes to fill a discharged stack, a rapidly increasing demand can not be covered in such a manner, or only with a plurality of stacks which are kept active.
A selective discharge of stacks without a flow for avoiding self-discharge resulting in heat generation is also disclosed in JP 2006313691 A1. Here, while the flow is stopped, an idle load strictly for the reduction of the electrical energy is electrically coupled to avoid an excessive temperature increase, however, without focusing on the effectiveness of the arrangement.
According to JP 61218076 B, a lead-acid battery in parallel to a flow battery is proposed to provide a buffer for short-term variations of the electricity demands while the flow battery takes over the long-term supply. U.S. Pat. No. 4,797,566 B describes in a similar manner that the flow batteries provide an “equalization” voltage for the lead-acid battery and also additional storage capacity for the whole system. It is also described that the flow battery—or a stack thereof—can be used as end cell for the lead-acid battery, i.e. arranged in series.
Thus, it was the object of the present invention to propose a system that ensures a fault-free and functionally reliable operation of an energy supply system on the basis of a redox flow battery while achieving the best possible efficiency.